Solid-state thin-film batteries are typically formed by stacking thin films on a substrate in such a way that the films cooperate to generate a voltage. The thin films typically include current collectors, a positive cathode, a negative anode, and an electrolyte film. The cathode and the electrolyte can be deposited utilizing a number of vacuum deposition processes, including physical vapor deposition, which includes evaporation and sputtering. Other methods of deposition include chemical vapor deposition and electroplating.
In a thin-film battery configuration, the cathode layer is separated from the anode layer by the insulating layer of solid electrolyte material. This electrolyte layer provides two functions. The first function is to conduct the electrochemically active ions between the cathode and the anode. The second function is to prevent the direct exchange of electrons between the cathode and the anode so that the electronic current becomes available only in the external circuit. In the case of a lithium based battery which comprises, for example, lithium (metal anode) batteries, lithium ion (anode) batteries, and lithium-free (anode) batteries, the electrochemically active ion that is exchanged is the Lithium+1 ion or the Li+ ion. In U.S. Pat. No. 5,597,660 to John B. Bates, Jan. 28, 1997, it is reported, “Most critical to battery performance is the choice of electrolyte. It is known that the principal limitation on recharge ability of prior batteries is failure of the electrolyte. Battery failure after a number of charge-discharge cycles and the loss of charge on standing is caused by reaction between the anode and the electrolyte, e.g. attack of the lithium anode on the lithium electrolyte in lithium batteries.” (Bates, column 2, lines 10-16).
The use of lithium phosphorus oxynitride is well known in the literature as LIPON and suitable for the formation of the solid thin film electrolyte layer in such devices. See, for instance, U.S. Pat. No. 5,569,520, Apr. 30, 1996 and U.S. Pat. No. 5,597,660, Jan. 28, 1997, issued to John B. Bates and N. J. Dudney. The resulting LIPON film was found to be stable in contact with the lithium anode. Perhaps as important, Bates et al. reported that incorporation of nitrogen into lithium orthophosphate films increased their Li+ ion conductivity up to 2.5 orders of magnitude.
Both U.S. Pat. Nos. 5,569,520 and 5,597,660 disclose the formation of amorphous lithium phosphorus oxynitride electrolyte films deposited over the cathode by sputtering Li3PO4, lithium orthophosphate, in a nitrogen atmosphere. Both patents teach that the targets were prepared by cold pressing of lithium orthophosphate powder followed by sintering of the pressed disc in air at 900° C. In each patent, deposition of a 1 micron thick film was carried out over a period of 16-21 hours at an average rate of 8-10 Angstroms per minute. The resulting film composition was LixPOyNz where x has approximate value 2.8, while 2y+3z equals about 7.8, and z has an approximate value of 0.16 to 0.46.
There were two shortcomings with regard to the sputter target disclosed by Bates, et al. One was the low rate of deposition. In part, the low deposition rate is a result of an inherent low sputter rate of a low density target material. In addition, high sputter rates also require high sputter power, which was not, or could not be applied. The other shortcoming is the presence of impurity phases in the lithium orthophosphate target material. These impurity phases can cause plasma instability, as reported by other workers for other sputter target materials. Also, the impurity phases that were likely present in the method taught by Bates could have weakened the sputter target and caused target cracking at higher sputter powers. Hence, high power and high rate deposition were not often possible with sputter targets having these impurities in the target material.
High density ceramic bodies or tiles of the lithium orthophosphate material suitable as sputtering target have been demonstrated by at least two methods: sintering and hot pressing. However, due to the large number of known oxides of phosphorus, targets that are commercially available have been shown by the present research and investigation to be rich in at least one or more impurity phases, such as Li4P2O7, which is deficient in lithium oxide (Li2O) as informally described by L4P2O7=Li3PO4●LiPO3 where LiPO3=Li3PO4−Li2O. Such compound and concentration variations of the impurity phases inside the parent material Li3PO4 due to presently available commercial manufacturing methods cause undesirable variations in the properties of the sputter target. These impurities cause plasma instability and target damage. For instance, they weaken the mechanical integrity of the sputter target, which is then prone to flaking and cracks. A weakened target can lead to particle generation, which, in turn, is built into the deposited film as defects. Impurities also weaken the sputter target through the formation of separated or agglomerated regions of higher or lower physical properties, such as density, elastic modulus, or color. At a given sputter power level these regions exhibit different sputter rates and sputtered composition compared with the surrounding sputter target areas. This scenario results in off-stoichiometric and non-uniform films. In particular, the impurity Li4P2O7 is deficient in lithium (oxide), which causes the deposited lithium phosphorus oxynitride film to be deficient in lithium. In that case, the Bates patent describes x of LixPOyNz equal to 2.8, and this patent is not specific regarding the actual ratio of lithium to phosphorus. Due to these process variations and defects caused by multi phase sputter targets, the deposited films display particle defects that typically result in electrical shorting of the thin-film battery. The same holds true if LIPON was used as the dielectric in a capacitor. The deposited films also show variation in chemical composition and poor uniformity. As a result, solid state batteries containing the subject LIPON electrolyte sputtered from multi phase lithium orthophosphate targets have poor yields and very low manufacturing rates. Furthermore, such films have not been practically manufacturable and therefore have remained only a scientific and engineering curiosity. Hence, mass produced batteries, typical of other vacuum thin film manufactured products such as semiconductor chips or LCD display panels, have not been made available for use with solid state batteries that first require generally defect free, uniform LIPON electrolyte films.
Lithium phosphorus oxynitride films must be chemically inert to the other layers present in thin film solid state batteries, capacitors and memory devices. Thin film batteries equipped with such an electrolyte are known to offer many benefits of high recharge cycle life, low impedance for fast charging and discharging, and high temperature operation such as 150° C. or even higher. Thin film batteries using lithium phosphorus oxynitride electrolytes can be made very small and thin, while providing high energy storage density when configured with thin film lithium cobalt oxide cathode layers.
Although, experimental batteries fabricated with lithium phosphorus oxynitride electrolyte layer have been reported, no commercial devices are generally available today due to the difficulty in sputtering from a lithium orthophosphate ceramic sputter target material formed by prior art methods. The industry has difficulty producing commercial thin film batteries with a lithium phosphorus oxynitride thin-film electrolyte for two major reasons. The first reason is that efficient sputtering of pure materials to form films or layers on a substrate cannot be accomplished economically from low density targets. The second reason is that sputter targets including more than one phase possess physical properties such as strength, elastic modulus, hardness, chemical composition, thermal conductivity, dielectric strength and even color that vary widely over the target surface. At a given power level the different phases of the target material will sputter at different rates, leading to non uniform erosion of the sputter target and non-uniform properties of the sputtered film. Moreover, commercial lithium orthophosphate targets evaluated were found to contain one or more impurity phases as discussed further below.
U.S. Pat. No. 5,435,826 by M. Sakakibara and H. Kikuchi discloses a method of forming a dense, single phase sputtering target of indium-tin oxide for sputtering an indium tin oxide layer or film by a particular method of sintering under particular high temperature and time conditions. In the '826 patent, a single phase sputter target having a density of 93% or more while containing a second impurity phase with a concentration of less than 10% is discussed. Sakakibara et. al. discloses the plasma instabilities that arise with multiphase targets as well as the high quality oxide film that can be made from the sputter target having both high density and high single phase composition.
Accordingly, there remains is a need for uniform high density commercial lithium orthophosphate targets. Therefore, there is also a need for a method of forming a dense, single phase sputter target of lithium orthophosphate that allows for deposition of a high quality LIPON layer at high rates of deposition.